1. Field of the Invention
The present invention relates to the so-called pulse-assisted magnetic-field-modulation-recording-system magneto-optical recording method and apparatus for recording information by applying a modulation magnetic field corresponding to the information by a magnetic head while irradiating a magneto-optical recording medium with a laser beam emitted like a pulse.
2. Related Background Art
In the case of a magnetic-field-modulation magneto-optical recording apparatus, information is recorded in a magneto-optical disk by arranging an optical head and a magnetic head so as to interpose the magneto-optical disk between the heads and applying a magnetic field whose polarities are reversed in accordance with a recording signal generated by the magnetic head to the magneto-optical disk while irradiating the magneto-optical disk with a laser beam emitted from the optical head.
The magnetic-field-modulation recording system is roughly divided into two types. One of the types is a DC-magnetic-field-modulation recording system for recording data by DC-emitting a laser beam and applying a magnetic field to be modulated corresponding to a recording signal to a magneto-optical recording medium from a magnetic head, and the other one of them is the so-called pulse-assisted magnetic-field-modulation recording system for recording data by emitting a laser beam like a pulse to optimize a temperature rise and a temperature distribution and applying a magnetic field to be modulated corresponding to a recording signal to a magneto-optical recording medium from a magnetic head. The latter system is recently noticed because the S/N of a reproduction signal can be further improved.
In this case, polarities of a magnetic field generated by a magnetic head are changed at the timing at which record data is modulated.
In the case of an optical head, however, the light-emitting period and light-emitting phase are controlled synchronously with a clock signal whose pulse width is set to a value smaller than the modulation period of a magnetic field generated by a magnetic head. In the case of the pulse-assisted magnetic-field-modulation recording, a phase relation between a laser pulse and a modulation magnetic field generated by a magnetic head is important and the laser pulse and modulation magnetic field greatly influence a signal quality. Moreover, to realize a high density and a high speed, the change speed (hereafter referred to as “switching speed”) of magnetic-field polarities of a modulation magnetic field is important.
The pulse-assisted magnetic-field-modulation recording is a recording system in which a pulse laser beam is irradiated to a magneto-optical recording medium, the magneto-optical recording medium is raised in temperature, magnetization is decided corresponding to the polarity of an applied magnetic field while the temperature of the magneto-optical recording medium is lowered, and a recording mark is formed. During the temperature lowering, if an applied magnetic field expands to the switching region of a modulation magnetic field and a magnetic field is not applied at a sufficient intensity, the quality of a reproduction signal is extremely deteriorated. Therefore, in the case of a magneto-optical recording apparatus, the laser-beam irradiation timing to the change timing of polarities of a magnetic field generated by a magnetic head is set to a timing at which optimum recording can be performed. For example, it is disclosed in Japanese Patent Application Laid-Open No. 2000-40273 to record a test signal in a recording medium by changing time phases of a laser pulse and a modulation magnetic field for a predetermined recording power, quantify the quality of a reproduction signal obtained from the test signal, and set a time phase corresponding to a test signal whose quality for reproduction is the best as an optimum phase.
A request for further improving the recording density of a magneto-optical recording medium to realize a large capacity has been raised in recent years and it is requested to decrease the pitch between recording tracks and downsize a recording mark.
Downsizing of a recording mark can be settled by accelerating a modulation magnetic field. At the same time, however, there is a problem that it is difficult to secure a sufficient phase margin and thereby, the quality of a reproduction signal is deteriorated. This point is described below in detail.
As shown in FIG. 4, the period Tw-Ts obtained by subtracting a modulation-magnetic-field polarity change speed (hereafter referred to as modulation-magnetic-field switching speed) Ts from a laser-pulse modulation cycle Tw serves as a substantial relative phase margin between a laser pulse and a modulation magnetic field. For example, when the modulation cycle Tw is set to 25 ns and the modulation-magnetic-field switching speed Ts is set to 15 ns, the phase margin between the laser pulse and modulation magnetic field becomes approx. 10 ns. Moreover, when raising a modulation-magnetic-field frequency in order to increase a density and speed, the period in which a magnetic-field intensity is saturated decreases and the phase margin between the laser pulse and modulation magnetic field is further decreased.
Furthermore, as shown in FIG. 5, when the recording power changes, the phase relation between the laser pulse and modulation magnetic field is also changed by changing a raised temperature and temperature distribution and further changing a magnetic-domain forming position. The same is also applied to a case in which not a recording power but a medium temperature is changed. Therefore, the phase margin 10 ns in the above example is further decreased.
When the above phase margin cannot be sufficiently secured, a phase shift exceeding the phase margin between a laser pulse and a modulation magnetic field may almost certainly occur due to a phase shift caused by the fluctuation of electrical components constituting an apparatus, a phase shift caused by the temperature characteristic of an electric system, or a phase shift caused due to environmental change such as temperature and thereby a signal quality is deteriorated.
The technique disclosed in Japanese Patent Application Laid-Open No. 2000-40273 makes it possible to set a time phase securing a sufficient phase margin with respect to a predetermined recording power. However, the recording-power dependency or cross-write characteristic of a recording medium is not considered at all. Therefore, a signal quality is deteriorated because the phase relation is changed due to the fluctuation of a medium temperature or change of recording powers.